Lipoprotein(a) ("Lp(a)") was first identified by Blumberg, B. S., et al. (1962) J. Clin. Invest. 41: 1936-1944 , and Berg, K. (1963) Acta Pathol. 59: 369-382. The structure of Lp(a) resembles that of low-density lipoprotein ("LDL") in that both share a lipid apoprotein composition, mainly apolipoprotein B-100 ("apo B"), the ligand by which LDL binds to the LDL receptors present on the interior surfaces of arterial walls. The unique feature of Lp(a) is an additional glycoprotein, designated apoprotein(a), apo(a), which is linked to apo B by disulfide groups. The cDNA sequence of apo(a) shows a striking homology to plasminogen, with multiple repeats of kringle 4, one kringle 5, and a protease domain. The isoforms of apo(a) vary in the range of 300 to 800 kDa and differ mainly in their genetically determined number of kringle 4 structures. McLean, J. W., et al. (1987) Nature 300: 132-137. Apo(a) has no plasmin-like protease activity. Eaton, D. L., et al., (1987) Proc. Natl Acad. Sci. USA, 84: 3224-3228. Serine protease activity, however, has been demonstrated. Salonen, E., et al. (1989) EMBO J. 8: 4035-4040. Like plasminogen, Lp(a) has been shown to bind to lysine-sepharose, immobilized fibrin and fibrinogen, and the plasminogen receptor on endothelial cells. Harpel, P.C., et al. (1989) Proc. Natl. Acad. Sci. USA 86:3847-3851; Gonzalez-Gronow, M., et al. (1989) Biochemistry 28: 2374-2377; Miles, L. et al. (1989) Nature 339: 301-302; Hajjar, K. A., et al. (1989) Nature 339: 303-305. Furthermore, Lp(a) has been demonstrated to bind to other components of the arterial wall like fibronectin and glycosaminoglycans. The nature of these bindings, however, is poorly understood.
Essentially all human blood contains lipoprotein(a); however, there can a thousand-fold range in its plasma concentration between individuals. High levels of Lp(a) are associated with a high incidence of cardiovascular disease. Armstrong, V. W., et al. (1986) Atherosclerosis 62: 249-257; Dahlen, G., et al. (1986) Circulation 74: 758-765; Miles, L. A., et al. (1989) Nature 339: 301-302; Zenker, G., et al. (1986). Stroke 17: 942-945 (The term occlusive cardiovascular disease will be used hereafter as including all pathological states leading to a narrowing and/or occlusion of blood vessels throughout the body, but particularly atherosclerosis, thrombosis and other related pathological states, especially as occurs in the arteries of the heart muscle and the brain.)
For some time, general medical practice has focused on the role of LDL, the so called "bad cholesterol," in occlusive cardiovascular disease. A great many studies have been published ostensibly linking occlusive cardiovascular disease with elevated levels of LDL. As a result, most therapies for the treatment and prevention of arteriosclerosis rely on drugs and methods for the reduction of serum levels of LDL's. Such therapies have had mixed results. The efficacy of such approaches to the problem of occlusive cardiovascular disease continues to be major source of debate.
There exists therefore a need for a drug therapy for reducing the binding of Lp(a) to vessel walls, for reducing the overall level of Lp(a) in the circulatory system and for promoting the release of existing deposits of Lp(a) on vessel walls.